Chapter 6: Effect of the Current

Chapter 6: Effect of the CurrentChapter 6: Effect of the Current

Ch6. Effect of the currentCh6. Effect of the current

• A current (mass of water) is several hundred times denser A current (mass of water) is several hundred times denser than air / generates forces of great magnitude than air / generates forces of great magnitude

• In open sea a ship is handled in the same way with or In open sea a ship is handled in the same way with or without currentwithout current

• The ship in a current is being carried away relative to fixed The ship in a current is being carried away relative to fixed objects (buoys, anchored vessels, obstructions…)objects (buoys, anchored vessels, obstructions…)

• Prior to start any manœuvre: assess tidal strenght and Prior to start any manœuvre: assess tidal strenght and direction in order to ascertain sufficient spacedirection in order to ascertain sufficient space

Ch6. Effect of the current / Position of Pivot PointCh6. Effect of the current / Position of Pivot Point

• Current acts on underwater Current acts on underwater portion of shipportion of ship

• When sailing with current: When sailing with current: ground speedground speed = = speed in speed in

waterwater + speed of current + speed of current

• When sailing against current: When sailing against current: ground speed = speed in ground speed = speed in

water – speed of currentwater – speed of current

Ch6. Effect of the current / Position of Pivot PointCh6. Effect of the current / Position of Pivot Point

Steering in the current:Steering in the current:

• Moving in the current:Moving in the current:– Faster: pivot point near bow, good steeringFaster: pivot point near bow, good steering– Slower: same as backing , pivot point near stern , unstable Slower: same as backing , pivot point near stern , unstable

shipship

• Moving against the current:Moving against the current:– Pivot point in bow quarters, good steering, stable shipPivot point in bow quarters, good steering, stable ship

Ch6. Effect of transverse currentCh6. Effect of transverse current

In beam current: ship must compensate the effect of In beam current: ship must compensate the effect of the currentthe current

Ch6. Effect of transverse currentCh6. Effect of transverse current

Ship dead in the water:Ship dead in the water:

• Force exerted by the current is very largeForce exerted by the current is very large• Proportional to exposed underwater surface of ship Proportional to exposed underwater surface of ship

(LxT) and to squared velocity of current V²(LxT) and to squared velocity of current V²• Force Force F = C x L x T x V²F = C x L x T x V²• Example: tanker of 260m LenghtExample: tanker of 260m Lenght

CurrentCurrent Ballast Ballast / 7m/ 7m

Loaded Loaded 14m14m

1 knot1 knot 30t30t 60t60t

2 knots2 knots 120t120t 240t240t

3 knots3 knots 270t270t 540t540t

Ch6. Effect of the current / Working in a tideCh6. Effect of the current / Working in a tide

When tide flows across a berth , it can be used to:When tide flows across a berth , it can be used to:– Improve slow speed controlImprove slow speed control– Create lateral motionCreate lateral motion

• Always better to stem the tideAlways better to stem the tide

Ch6. Effect of the current / Tide from aheadCh6. Effect of the current / Tide from ahead

-Short kicks ahead to Short kicks ahead to maintain headway through maintain headway through the water and keep pivot the water and keep pivot point forward with little point forward with little speed over groundspeed over ground

-Good steering leverGood steering lever

-Good control over the Good control over the shipship

- This is know as - This is know as « stemming the thide »: « stemming the thide »: ship’s speed bigger than ship’s speed bigger than current speed.current speed.

Ch6. Effect of the current / Tide from asternCh6. Effect of the current / Tide from astern

• Most unsatisfactory Most unsatisfactory situationsituation

• Extremely difficult to keep Extremely difficult to keep positive control of the positive control of the shipship

• To keep pivot point ahead, To keep pivot point ahead, ship must be running at a ship must be running at a speed over ground much speed over ground much higher than speed of the higher than speed of the tide: tide: far too fastfar too fast

• To reduce speed: engine To reduce speed: engine astern / astern / pivot point moves pivot point moves after + transverse thrustafter + transverse thrust

Ch6. Effect of the current / Working across a tideCh6. Effect of the current / Working across a tide

1.1. Balance ship’s speed throughBalance ship’s speed through the water and tidal streamthe water and tidal stream

2. Create sideway (lateral 2. Create sideway (lateral motion):motion):

- use rudder angle alone or with - use rudder angle alone or with kick aheadkick ahead

- current comes onto opposite - current comes onto opposite bowbow

- resultant of 2 vectors brings - resultant of 2 vectors brings vessel towards the berth vessel towards the berth (crablike)(crablike)

- to stop drift: bring ship’s head - to stop drift: bring ship’s head into the tideinto the tide

Ch6. Effect of the current / Working across a tideCh6. Effect of the current / Working across a tide

When working across a tide:When working across a tide:• Be patientBe patient• Never rush the manoeuvreNever rush the manoeuvre• Always put the tide fine on theAlways put the tide fine on the bowbow• Don’t put the tide too far aroundDon’t put the tide too far around on the bow:on the bow: - - good lateral motiongood lateral motion - difficult to bring ship back into- difficult to bring ship back into the tidethe tide

Ch6.Ch6. Effect of the current / VectorsEffect of the current / Vectors

Ship’s head into the tideShip’s head into the tide

• Ship’s speed through the Ship’s speed through the water = speed of tidewater = speed of tide

• No sideways drift :No sideways drift :

• αα = 0 (angle between ship’s = 0 (angle between ship’s heading and current)heading and current)


• αα = 60° and V = S = 60° and V = S• V1 V1 < S → ship is backing< S → ship is backing• V and S produce R (drift)V and S produce R (drift)

• Conclusion: Conclusion: - If V= S , the ship will - If V= S , the ship will berth astern of hisberth astern of his

positionposition-V has to be increasedV has to be increased in order to berth at thein order to berth at the good place (see next good place (see next slide).slide).


To keep position: V must beTo keep position: V must beincreased until V1 = Sincreased until V1 = SThen will R coincide with V2Then will R coincide with V2And vessel drifts perpendicularAnd vessel drifts perpendiculartowards the berth.towards the berth.

For For αα = 22°5: = 22°5:a)a) S = 2’ → V = 2’3 / R = 0’8S = 2’ → V = 2’3 / R = 0’8b)b) S = 3’ S = 3’ → → V = 3’3 / R = 1’2V = 3’3 / R = 1’2

For For αα = = 45°:45°:a)a) S = 2’ S = 2’ → V = 2’8 / R = 2’→ V = 2’8 / R = 2’b)b) S = 3’ → V = 4’3 / R = 3’S = 3’ → V = 4’3 / R = 3’

Ch6. Effect of current on manoeuvrabilityCh6. Effect of current on manoeuvrability

Turning in the currentTurning in the current a)a) With the current, the ship makes a wide swingWith the current, the ship makes a wide swingb)b) Against the current it makes a tight swingAgainst the current it makes a tight swing

Ch6. Swinging on an anchorCh6. Swinging on an anchor

A ship with a following tideA ship with a following tideswing through 180° to stem theswing through 180° to stem thetide prior to proceeding to itstide prior to proceeding to itsberth.berth.

In a narrow waterway the shipIn a narrow waterway the shipSwings on an anchor, keepingSwings on an anchor, keepingA tight control over the position.A tight control over the position.

This is only possible if the bottomThis is only possible if the bottom is clear of obstructionsis clear of obstructions

Ch6. Effect of the current / Swinging on an anchorCh6. Effect of the current / Swinging on an anchor

This manoeuvreThis manoeuvre depends on experience and skill and:depends on experience and skill and:• Depth of waterDepth of water• UKC (Under Keel Clearance)UKC (Under Keel Clearance)• Strenght of the currentStrenght of the current• Type of bottomType of bottom• Type of engine power availableType of engine power available• Size of the shipSize of the ship• Amount of room available for turnAmount of room available for turn

Ch6. Effect of the current / Bends in a tidal RiverCh6. Effect of the current / Bends in a tidal River

The tide may be of different strenghts : rapidly on the The tide may be of different strenghts : rapidly on the outside but weaker on the inside of the bendoutside but weaker on the inside of the bend

Ch6. Effect of the current / Following tideCh6. Effect of the current / Following tide

The strong tide is working on the stern with the pivot pointThe strong tide is working on the stern with the pivot pointforward: good turning lever and strong turning force.forward: good turning lever and strong turning force. Anticipation + kick ahead and counter rudderAnticipation + kick ahead and counter rudder

Ch6. Effect of the rudder / Following TideCh6. Effect of the rudder / Following Tide

• A ship can react violently and rapidly A ship can react violently and rapidly to this force : never underestimate.to this force : never underestimate.

• Better to keep more to the outside of Better to keep more to the outside of the bend: the ship is always in the area the bend: the ship is always in the area of stronger following tide.of stronger following tide.

Ch6. Effect of the rudder / Tide from aheadCh6. Effect of the rudder / Tide from ahead

When a large ship negotiates a bend in a channel with currentWhen a large ship negotiates a bend in a channel with currentfrom ahead:from ahead:Better to keep to the inside so that the bow does not enter the Better to keep to the inside so that the bow does not enter the area of stronger current during the turn.area of stronger current during the turn.

This side is often the shallow side as wellThis side is often the shallow side as well

Ch6. Effect of the rudder / Tide from aheadCh6. Effect of the rudder / Tide from ahead

In this position, the ship’s bow is influenced by the strongIn this position, the ship’s bow is influenced by the strongTide : the turning moment opposes the intended turn.Tide : the turning moment opposes the intended turn.Risk of grounding Anticipation with helm and powerRisk of grounding Anticipation with helm and power

Ch6. Effect of current / Rapid changes in tidal directionCh6. Effect of current / Rapid changes in tidal direction


A ship can pass close to shallow areas or man madeA ship can pass close to shallow areas or man madestructures where the tide changes rapidly in direction.structures where the tide changes rapidly in direction.If the ship proceeds at slow speed this can have seriousIf the ship proceeds at slow speed this can have seriousConsequences for the handling.Consequences for the handling.


Ch6. Effect of the current / Restricting the tidal flowCh6. Effect of the current / Restricting the tidal flow

The tide is forced to flow faster between ship and berth:The tide is forced to flow faster between ship and berth: a low pressure appears and the ship is sucked towards the a low pressure appears and the ship is sucked towards the quay.quay.

Avoid acute angles with a jetty, even with Avoid acute angles with a jetty, even with weak tidesweak tides

Ch6. Effect of the current / Tidal forcesCh6. Effect of the current / Tidal forces

Force of the tide depends onForce of the tide depends on::

» Draft and depth of waterDraft and depth of water» Ship’s bow configurationShip’s bow configuration» Velocity of the tideVelocity of the tide» Under keel clearanceUnder keel clearance

Force of tide can vary with the depthForce of tide can vary with the depth: a tidal difference: a tidal differenceOf up to 2.5knots over a depth of 5 meter is possible. Of up to 2.5knots over a depth of 5 meter is possible.

Published « Tidal Stream » is sometimes inaccurate.Published « Tidal Stream » is sometimes inaccurate.


• Force of the tide: directly proportional to the square of the Force of the tide: directly proportional to the square of the velocityvelocity

• A small increase in speed means an enormous increase in A small increase in speed means an enormous increase in force exerted upon a shipforce exerted upon a ship


UKCUKC: blocking effect of a vessel when the UKC is reduced / : blocking effect of a vessel when the UKC is reduced / the tide cannot flow under and is forced to flow aroundthe tide cannot flow under and is forced to flow around

Ch6. Effect of the current / Tidal force at anchorCh6. Effect of the current / Tidal force at anchor

50.000DWT tanker at anchor / 5 knots tide / depth/draft ratio: 3.0:50.000DWT tanker at anchor / 5 knots tide / depth/draft ratio: 3.0:The tidal force = 19 tonnes (exerted on the windlass)The tidal force = 19 tonnes (exerted on the windlass)

Ch6. Effect of the current / Tidal force at anchorCh6. Effect of the current / Tidal force at anchor

50.000DWT tanker at anchor / 5 knots tide / depth/draft ratio:1.1 50.000DWT tanker at anchor / 5 knots tide / depth/draft ratio:1.1 due to falling tide:due to falling tide:Tidal force increases three times to 64 tonnes and can exceed theTidal force increases three times to 64 tonnes and can exceed theHolding power of the anchor Holding power of the anchor → → ship will drag.ship will drag.

Longitudinal forces on tanker at anchor Longitudinal forces on tanker at anchor W

ind 50knots / Tide: 5knots W

ind 50knots / Tide: 5knots

Ch6. Effect of the current / Lateral tidal forcesCh6. Effect of the current / Lateral tidal forces

When ship moored or held with the tide on the beamWhen ship moored or held with the tide on the beamwith a small UKC: lateral force created can be enormouswith a small UKC: lateral force created can be enormous

Ch6. Effect of the current / Lateral tidal forcesCh6. Effect of the current / Lateral tidal forces

Example 1:Example 1: Ship of 280.000 dwt / draft Ship of 280.000 dwt / draft

22m22mCurrent: 1knot on the beamCurrent: 1knot on the beamDepth-draft ratio: 1.05Depth-draft ratio: 1.05Total lateral force = 328 Total lateral force = 328

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Example 2:Example 2:Same shipSame shipCurrent: 1,5 knots on the Current: 1,5 knots on the

beambeamTotal lateral force = 700 Total lateral force = 700

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Chapter 6: Effect of the Current